1. Field of the Invention
The invention relates to a projection type liquid crystal display unit, and more particularly to an improvement in a frame supporting a liquid crystal light bulb therewith for prevention reflection of a light therefrom.
2. Description of the Related Art
Recently, various projection type liquid crystal display units such as a liquid crystal projector have been developed. A projection type liquid crystal display unit is grouped into a light-transmission type and a light-reflection type, and further grouped into a single plate type and a three-plates type in dependence on the number of liquid crystal light bulbs to be used therein.
Hereinbelow is explained a light-transmission and single plate type liquid crystal display unit with reference to FIG. 2.
The illustrated liquid crystal display unit is comprised of a light source 1, a converging lens 2, a first polarizing plate 3a, a liquid crystal light bulb 4, a second polarizing plate 3b, a projection lens 5, and a screen 6.
In operation, a natural light emitted from the light source 1 passes through the converging lens 2, and is converted into a linearly polarized light by the first polarizing plate 3a. The thus converted linearly polarized light enters the liquid crystal light bulb 4, and is modulated in the liquid crystal light bulb 4 as variation in polarization in accordance with an image signal. The light leaving the liquid crystal light bulb 4 passes through the second polarizing plate 3b, and is projected onto the screen in an increased size by the projection lens 5.
Hereinbelow is explained a light-transmission and three-plate type liquid crystal display unit with reference to FIG. 6.
The illustrated liquid crystal display unit is comprised of a light source 1, an optical system 21 for converting a light into a polarized light, two dichroic mirrors 22, four all-reflection mirrors 23, converging lenses 24, first polarizing plates 25a, second polarizing plates 25b, light liquid crystal bulbs 26, a dichroic prism 27, a projection lens 28, and a screen 29.
In operation, natural lights emitted from the light source 1 are converted into linearly polarized lights having the same forwarding direction and polarizing direction, by the optical system 21. The thus converted linearly polarized lights are separated into blue, green and red lights by an optical separation system comprised of the two dichroic mirrors 22 and the all-reflection mirrors 23. The blue, green and red lights pass through the associated converging lenses 24 and first polarizing plates 25a, and then, illuminate the associated liquid crystal light bulbs 26.
The linearly polarized lights entering the liquid crystal light bulbs 26 are modulated in the liquid crystal light bulbs 26 as variation in polarization in accordance with an image signal. The lights leaving the liquid crystal light bulbs 26 enter the associated second polarizing plates 25b. A portion of the lights displaying black is absorbed into the second polarizing plates 25b, whereas a portion of the lights displaying white passes through the second polarizing plates 25b. 
After passing through the second polarizing plates 25b, optical images formed on the liquid crystal light bulbs 26 are combined into a single optical image at the dichroic prism 27. The thus produced optical image is projected onto the screen 29 in an increased size through the projection lens 28.
Each of the liquid crystal light bulbs 26 is comprised of a liquid crystal display panel and a frame supporting the liquid crystal display panel therewith. The liquid crystal display panel is supported by the frame by being sandwiched between the frame, and is fixed to a body of the liquid crystal display unit through the frame.
In the above-mentioned projection type liquid crystal display unit, a light leaving a liquid crystal light bulb sometimes returns back to the liquid crystal light bulb because of reflection, refraction, scattering, diffraction, divergence, deviation in an optical axis and/or aberration in the parts constituting the liquid crystal display unit, before the light is projected onto a screen as an image.
If such a light once having left a liquid crystal light bulb and returned back thereto were reflected or scattered at a surface of a frame of a liquid crystal light bulb, the light would stray into an optical path having another optical data, resulting in degradation in display quality, such as reduction in contrast of a projected image, a double image or ghost, and/or local non-uniformity in brightness.
In order to avoid the above-mentioned problem, a frame of a liquid crystal light bulb is usually composed of metal to which a special coating is applied.
FIGS. 1A and 1B illustrate cases in which a light is reflected to a frame of a liquid crystal light bulb from other parts constituting the liquid crystal display unit.
A body of a projection type liquid crystal display unit is usually coated at an inner wall thereof with black painting, and an optical part is usually coated at a surface thereof with a film which prevents light reflection.
However, as illustrated in FIG. 1A, a light may be reflected many time between a an all-reflection mirror 51 and a converging lens 52, and reach a frame of a liquid crystal light bulb. 53. Then, the light is reflected at the frame and subsequently a projection lens 56, and enters a dichroic prism 55. Thus, the light strays into another image data.
As an alternative, as illustrated in FIG. 1B, a light may be wrongly reflected at a dichroic prism 55, and resultingly, reaches a frame of a liquid crystal light bulb 53, if lights emitted from a light source are not parallel with one another, and/or an optical axis is deviated because of non-uniformity in a gap in a liquid crystal panel.
In addition, a lamp used in a projection type liquid crystal display unit, such as a halogen lamp, a xenon lamp, a metal halide lamp or a high-pressure mercury lamp, and/or light absorption in parts having a small light-reflection rate generate heat in a body of a liquid crystal display unit, and resultingly, the body is kept at a high temperature inside. This causes convection current in the body. As a result, a light is scattered and/or reflected by dusts, and then, reaches a frame. The light is further scattered and/or reflected at the frame, resulting in that the light may enter another image data.
As mentioned above, reflection and/or scattering of a light cause degradation and non-uniformity in projected images.
As mentioned earlier, a liquid crystal light bulb in a projection type liquid crystal display unit is comprised of a liquid crystal display panel and a frame. A module structure of a liquid crystal display unit is grouped into two groups in dependence on whether positioning between a frame and a liquid crystal display panel is performed based on either a thin film transistor (TFT) substrate or an opposing substrate of the liquid crystal display panel. This is because that a thin film transistor substrate is usually arranged at a side at which a light leaves, in order to avoid the heat problem. Herein, the positioning between a frame and a liquid crystal display panel indicates positioning between a reference position defined at a frame or a body of a liquid crystal display unit and a display area or a center of a display area of a liquid crystal display panel.
FIG. 10 is an exploded perspective view of a liquid crystal light bulb.
Hereinbelow is explained how a liquid crystal display panel and a frame are positioned to each other, based on a thin film transistor substrate of the liquid crystal display panel, with reference to FIG. 10.
First, as illustrated in FIG. 10(a), a liquid crystal display panel 61 is positioned relative to a first frame 62. A flexible printing substrate 65 is connected to the liquid crystal display panel 61 for connecting the liquid crystal display panel 61 to an external circuit (not illustrated).
Then, as illustrated in FIG. 10(b), the liquid crystal display panel 61 is adhered to the first frame 62 through an adhesive.
Then, as illustrated in FIG. 10(c), a second frame 63 through which an incident light first passes is fixed to the first frame 62 such that the liquid crystal display panel 61 is sandwiched between the first and second frames 62 and 63.
If the first frame 62 were composed of molded resin, the first frame 62 would have a surface to which a light is much reflected. Hence, as illustrated in FIG. 10(d), a metal frame 64 is further fixed to the first frame 62. In addition, the metal frame 64 is necessary to be coated at a surface thereof with a coating for preventing light reflection.
Though the first frame 62 may be formed as a metal frame coated at a surface thereof with a coating for light reflection, it would be quite difficult to manufacture the metal frame with sufficient dimensional accuracy, and to position and fix the metal frame to the liquid crystal display panel 61.
The second frame 63 may be composed of molded resin or metal.
FIG. 11 is an exploded perspective view of a liquid crystal light bulb, similarly to FIG. 10.
Hereinbelow is explained how a liquid crystal display panel and a frame are positioned to each other, based on an opposing substrate of the liquid crystal display panel, with reference to FIG. 11.
First, a first frame 72 through which an incident light first passes is fixed to a liquid crystal display panel 71. A flexible printing substrate 74 is connected to the liquid crystal display panel 71 for connecting the liquid crystal display panel 71 to an external circuit (not illustrated).
Then, a second frame 73 is fixed to the first frame 72 such that the liquid crystal display panel 71 is sandwiched between the first and second frames 72 and 73.
The second frame 73 may be formed of a metal frame coated at a surface thereof with a coating for light reflection. However, in order to position the liquid crystal display panel relative to the first and second frames 72 and 73, based on an opposing substrate of the liquid crystal display panel 71, the opposing substrate would be required to have high accuracy in an outer shape which is not usually required to have.
The first frame 72 may be composed of molded resin or metal, but is usually composed of molded resin.
When the metal frame 64 is fixed to the first and second frames 62 and 63 both composed of molded resin, there would be caused a problem of an increased burden in designing a liquid crystal light bulb, due to a difference in a coefficient of linear expansion between the metal frame 64 and the first and second frames 62 and 63.
In addition, there are further caused problems of an increase in both the number of parts and fabrication costs.
Though the above-mentioned problems are explained as problems in a light-transmission type liquid crystal display unit, the same problems are also caused in a light-reflection type liquid crystal display unit.
Japanese Unexamined Patent Publication No. 4-104244 has suggested a projection type liquid crystal display unit. The liquid crystal display unit includes three liquid crystal display panels each of which is illuminated by red, green and blue lights, and synthesizes images formed on the three liquid crystal display panels, to project the synthesized images onto a screen. The liquid crystal display unit further includes a polarizing device to which an optical device on which an anti-reflective film is coated is coupled. The polarizing device is adhered to each of panel substrates positioned at opposite sides of the three liquid crystal display panels.
Japanese Unexamined Patent Publication No. 7-64070 has suggested a liquid crystal display device including a first electrode substrate having pixel electrodes arranged in a matrix and coplanar thin film transistors associated with the pixel electrodes, a second electrode substrate including an opposing electrode, and a liquid crystal layer sealed between the first and second electrode substrates. The liquid crystal display further includes a polarizing plate adhered to the first electrode substrate, and an anti-reflective layer formed on a surface of the polarizing plate at which an incident light leaves outwardly.
Japanese Unexamined Patent Publication No. 8-76081 has suggested a projection type liquid crystal display unit including a liquid crystal panel having an active matrix substrate on which semiconductor active devices are formed, an opposing substrate on which an opposing electrode is formed, and a liquid crystal layer sandwiched between the active matrix substrate and the opposing substrate. In operation, an image formed on the liquid crystal panel is projected onto a screen through a projection lens by illuminating the liquid crystal panel from the opposing substrate. The liquid crystal panel is designed to have a light-impermeable pattern for covering the semiconductor active device therewith in order to prevent an incident light from reaching directly to the semiconductor active device. A planar polarizing device is adhered to a surface of the active matrix substrate at which an incident light leaves, and is designed to have an anti-reflection layer at a surface at which a light leaves.
Japanese Unexamined Patent Publication No. 11-149071 has suggested a liquid crystal display device including a liquid crystal cell having two substrates and a liquid crystal layer sandwiched between the substrates, and a plate located in the vicinity of or making contact with the liquid crystal cell. The plate has a photoelastic coefficient having a characteristic relative to a temperature which characteristic is just inverse to the same of a photoelastic coefficient of the substrates. The plate compensates for non-uniformity in contrast.
However, the above-mentioned problems remain unsolved even in the above-mentioned Publications.